The present invention generally relates to speech detection circuits, and more particularly to a speech detection circuit which is suited for use in a control part of a mobile communication terminal.
In mobile communication terminals, it is desirable to extend the serviceable life of a built-in battery as long as possible. For this reason, a speech detection circuit is required to detect the existence of speech during communication and carry out a power save operation when no speech input exists.
FIG. 1 shows an example of a conventional speech detection circuit. In FIG. 1, a microphone 10 picks up speech and outputs an audio signal. An amplifier 20 amplifies the audio signal output from the microphone 10, and a bandpass filter 30 eliminates noise included in an output signal of the amplifier 20. A coupling capacitor C1 eliminates a D.C. component included in an output signal of the bandpass filter 30. R1 denotes an input resistance of an amplifier 12, and R2 denotes a feedback resistance of the amplifier 12. The resistances R1 and R2 together determine a gain of the amplifier 12.
A rectifying circuit 2 rectifies an output signal of the amplifier 12 and outputs a D.C. voltage. A comparator 3 compares the output signal of the rectifying circuit 2 with a reference level S, and supplies an output signal which controls the ON/OFF state of a switch 41. A capacitor C2 is connected in parallel to the switch 41, and a constant current source 42 is connected in series to a parallel circuit which is made up of the capacitor C2 and the switch 41. A shaping circuit 43 shapes an output voltage waveform of the capacitor C2. For example, a one-shot multivibrator is used as the shaping circuit 43.
Next, a description will be given of an operation of the conventional speech detection circuit shown in FIG. 1, by referring to FIG.2. In FIG. 2, (A) shows an input signal waveform at an output of the capacitor C1, (B) shows a rectified signal output from the rectifying circuit 2, and (C) shows an output signal OUT1 which is output from the shaping circuit 43.
First, the audio signal from the microphone 10 is passed through the amplifier 20 and the bandpass filter 30, and the D.C. component of the audio signal is eliminated by the capacitor C1 before being supplied to the amplifier 12. Hence, the signal shown in FIG. 2(A) is supplied to the amplifier 12 and is amplified with a gain G which is determined by the resistances R1 and R2. The output signal of the amplifier 12 is passed through the rectifying circuit 2 and is formed into the D.C. signal shown in FIG. 2(B).
The comparator 3 compares the D.C. signal shown in FIG. 2(B) with the reference level S. Hence, when the signal received via the capacitor C1 has the signal waveform shown in FIG. 2(A), the output signal level of the comparator 3 becomes high at a time t1 when the rectified (D.C.) signal shown in FIG. 2(B) falls below the reference level S. The switch 41 is switched from the closed state to the open state when the output signal level of the comparator 3 becomes high. In this case, however, FIG.2(A) shows the signal waveform before being amplified with the gain G in the amplifier 12, and thus, the the actual input signal level to the comparator 3 after the amplification with the gain G is (S-G) dBV as shown in FIG. 2(A).
When the switch 41 is opened, the charging of the capacitor C2 by the constant current source 42 starts. At a time t2 when the voltage at the capacitor C2 becomes greater than or equal to a predetermined value (for example, approximately two seconds after the time t1), the output signal OUT1 of the shaping circuit 43 undergoes a transition from a low level which indicates a normal operation in which no power save is made to a high level which indicates a power save operation.
When the amplitude of the signal becomes large again as shown in FIG.2(A), the output signal level of the comparator 3 becomes low at a time t3 when the input signal level (S-G) is exceeded. Hence, the switch 41 is switched from the open state to the closed state. As a result, the charge in the capacitor C2 is instantaneously discharged via the switch 41, thereby immediately changing the signal level of the output signal OUT1 of the shaping circuit 43 to the low level to indicate the normal operation. In other words, the operation returns to the normal operation from the power save operation.
However, according to the conventional speech detection circuit, the input signal level used for switching the operation from the normal operation to the power save operation and the input signal level used for switching the operation from the power save operation to the normal operation are (S-G) and are the same. For this reason, when the surrounding noise level becomes high, for example, the input signal level (S-G) is easily exceeded. As a result, there is a problem in that the operation returns to the normal operation although originally the power save operation should be continued. In other words, the operation is erroneously returned to the normal operation from the power save operation when the noise level is relatively high.